1. Field of the Invention
This invention relates to laser mirrors. More specifically, it relates to materials used for fabricating distributed Bragg reflector mirrors, and to distributed Bragg reflector mirrors made from those materials. Such distributed Bragg reflector mirrors are according to the principles of the present invention are suitable for use in vertical cavity surface emitting lasers (VCSELs).
2. Discussion of the Related Art
Vertical cavity surface emitting lasers (VCSELs) represent a relatively new class of semiconductor lasers. While there are many VCSEL variations, a common characteristic is that VCSELs emit light perpendicular to a semiconductor wafer's surface. Advantageously, VCSELs can be formed from a wide range of material systems to produce specific characteristics. In particular, material systems can be tailored to produce different laser wavelengths, such as 1550 nm, 1310 nm, 850 nm, 670 nm, and so on.
VCSELs include semiconductor active regions, distributed Bragg reflector (DBR) mirrors, current confinement structures, substrates, and contacts. Because of their complicated structure, and because of their specific material requirements, VCSELs are usually grown using metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE).
FIG. 1 illustrates a typical VCSEL 10. As shown, an n-doped gallium arsenide (GaAs) substrate 12 has an n-type electrical contact 14. An n-doped lower mirror stack 16 (a DBR) is on the GaAs substrate 12, and an n-type graded-index lower spacer 18 (a beneficially having a number of quantum wells, is formed over the lower spacer 18. A p-type graded-index top spacer 22 (another confinement layer) is disposed over the active region 20, and a p-type top mirror stack 24 (another DBR) is disposed over the top spacer 22. Over the top mirror stack 24 is a p-type conduction layer 9, a p-type GaAs cap layer 8, and a p-type electrical contact 26.
Still referring to FIG. 1, the lower spacer 18 and the top spacer 22 separate the lower mirror stack 16 from the top mirror stack 24 such that an optical cavity is formed. Because an optical cavity is resonant at specific wavelengths, the mirror separation is controlled to resonate at a predetermined wavelength (or at a multiple thereof). At least part of the top mirror stack 24 includes an insulating region 40 that is usually formed either by implanting protons into the top mirror stack 24 or by providing an oxide layer. The protons can be implanted, for example, in accordance with the teachings of U.S. Pat. No. 5,115,442, which is incorporated by reference. The oxide layer can be formed, for example, in accordance with the teachings of U.S. Pat. No. 5,903,588, which is incorporated by reference. The insulating region 40 defines a conductive annular central opening 42 that forms an electrically conductive path through the insulating region 40.
In operation, an external bias causes an electrical current 21 to flow from the p-type electrical contact 26 toward the n-type electrical contact 14. The insulating region 40 and the conductive central opening 42 confine the current 21 such that it flows through the conductive central opening 42 to the active region 20. Some of the electrons in the current 21 are converted into photons in the active region 20. Those photons bounce back and forth (resonate) between the lower mirror stack 16 and the top mirror stack 24. While the lower mirror stack 16 and the top mirror stack 24 are very good reflectors, some of the photons leak out as light 23 that travels along an optical path. Still referring to FIG. 1, the light 23 passes through the p-type conduction layer 9, through the p-type GaAs cap layer 8, through an aperture 30 in the p-type electrical contact 26, and out of the surface of the vertical cavity surface emitting laser 10.
It should be understood that FIG. 1 illustrates a common VCSEL structure, and that numerous variations are possible. For example, the dopings can be changed (say, by providing a p-type substrate 12), a wide range of material systems can be used, operational details can be tuned for maximum performance, and additional structures, such as tunnel junctions, can be added. However, the VCSEL 10 beneficially illustrates a useful, common, and exemplary VCSEL configuration.
While generally successful, VCSELs have problems. In particular, in some applications the prior art distributed Bragg Reflectors (DBR) are significantly less than optimal. To understand why this is so, it is beneficial to consider DBRs in more detail.
A DBR must be highly optically reflective and electrically conductive, and beneficially should be thermally conductive as well. When used in VCSELs, and particularly in long-wavelength VCSELs, a DBR must be very highly reflective to reduce optical losses to enable laser operation. Reflectivity is achieved by stacking material layers having different indexes of refraction, for example, alternating layers of AlAs and GaAs. Such stacked layers can produce an optical standing wave within the VCSEL.
While the optical performance of a DBR comprised of AlAs and GaAs is very good, to produce a VCSEL that emits a long wavelength it is beneficial to use an InP substrate. Unfortunately, because of the high degree of lattice mismatch between AlAs/GaAs and InP, it is very difficult to produce a high quality AlAs/GaAs DBR on an InP substrate. Furthermore, when attempting to do so, such as by implementing AlAs/GaAs layers that are thinner than the critical thickness (subsequently described), the group V source needs to be changed between AlAs/GaAs and InP growths (As-rich to P-rich).
The critical thickness refers to the growth thickness of an overlayer on a substrate at which the number of crystalline defects increases dramatically because of the lattice mismatch. If the overlayer and the substrate have similar crystalline structures the critical thickness is large, but if the overlayer and the substrate are highly mismatched the critical thickness is very small.
In addition to AlAs/GaAs material systems, other mirror material systems, including AlInGaAs, InGaAsP/InP, and GaAlAsSb/AlAsSb are known. Unfortunately, material systems comprised of more than two materials tend to have low thermal conductivity, which makes heat difficult to remove, and low refractive index differences, which means that many stacked layers are required. This increases fabrication costs and time. Thus, two component material systems, referred to as binary systems, are highly advantageous.
Therefore, a new material system suitable for use in VCSEL DBRs, particularly at long wavelengths, would be beneficial. Even more beneficial would be a new VCSEL DBR material system that has good thermal conductivity and that is comprised of material layers having a high index of refraction difference. Even more beneficial would be a new VCSEL DBR material system that has good thermal conductivity, that is comprised of binary material layers having high index of refraction differences, and that can be fabricated without changing the group V source during fabrication.